1. Field of the Invention
The present invention relates to a mass flowmeter of Coriolis type that generates a force known as Coriolis force acting in proportion to the mass flow rate of a fluid passing through a pipe by forcibly oscillating the pipe, and determining the mass flow rate by detecting a phase difference of the vibrations on the upstream side and on the downstream side of the pipe.
2. Description of the Background Art
A flow meter of Coriolis type forcibly oscillates with an oscillator a fluid pipe for passing a fluid to be measured, detects a phase difference of the vibrations generated on the upstream side and on the downstream side of a flow path in accordance with a flow rate of the mass of a fluid, and determines the mass flow rate from the phase difference (WO91/08448 (abstract)).
Here, prior to the description of the problem of the present invention, a structure, a principle, and the like of a mass flowmeter of Coriolis type will be described.
In FIG. 4, for example, a substantially U-shaped fluid pipe 1 forms a flow path of a measurement fluid 100 to be measured. The measurement fluid 100 is introduced from one end of the fluid pipe 1, and passes through a bent portion and a straight-pipe portion to be ejected from the other end.
Both ends of the substantially U-shaped fluid pipe 1 are fixed to a wall portion 101. When this is seen from a viewpoint of structural mechanics, the wall portion 101 serving as a supporting member supports the fluid pipe 1 such that both ends of the fluid pipe 1 will be fixed ends relative to the vibration generated by the oscillation of a oscillator 2 described below, that is, it will be similar to a structure where the fluid pipe 1 is supported in a cantilever state. As a result, as shown in the model view of FIG. 6A, the fluid pipe 1 can be warped with an axis located at both ends inserted into the wall portion 101, that is, at the fixed ends of the vibration by the oscillation or in its vicinity.
In FIG. 4, an oscillator 2 is disposed at an intermediate portion in the fluid pipe 1. The oscillator 2 is made of a permanent magnet (magnetic substance) 21 fixed to the fluid pipe 1 and an electromagnetic driving coil 22 fixed on a base 102. The permanent magnet 21 is inserted into the electromagnetic driving coil 22, and oscillates the fluid pipe 1 when an alternating current is passed through the electromagnetic driving coil 22 by an oscillation circuit 34 (FIG. 5). That is, the electromagnetic driving coil 22 is disposed to correspond to the permanent magnet 21, and the permanent magnet 21 performs reciprocating movement in an axial direction within the electromagnetic driving coil 22.
On the other hand, the fluid pipe 1 is provided respectively with first and second detectors 2A, 2B. That is, the first and second detectors 2A, 2B are arranged to be spaced apart from each other on the upstream side and on the downstream side along a flow path 10 of the fluid pipe 1. Each of the detectors 2A, 2B is made of a well-known electromagnetic pickup, and is constituted with a detected element (magnetic substance) 23 made of a permanent magnet and a detection coil 24 corresponding to the detected element 23. The velocity of the vibration which is one of the vibration states in the vertical direction of the fluid pipe 1 is detected by an electric power that is generated by mutual reciprocating movement of the detected element 23 within the detection coil 24.
In FIG. 5, a signal related to the velocity of the vibration that has been detected by the first detector 2A passes through a detection circuit 33 to be transmitted to a calculator 32 of a microcomputer 3 and to be transmitted to an oscillation circuit 34. The oscillation circuit 34 supplies to the electromagnetic driving coil 22 constituting the oscillator 2 an electric current that accords to the magnitude and the positive/negative direction of the signal related to the velocity of the vibration that has been detected by the first detector 2A such that positive feedback may be applied. At this time, positive feedback is applied at a specific natural frequency of the fluid pipe 1 which is a frequency at which the vibration is difficult to be damped against the oscillation to generate an oscillated state, whereby the vibration at the basic natural frequency can be maintained at a constant level. Here, by adjusting the arrangement of the first detector 2A, the positive/negative direction of the positive feedback, and the like, the vibration of the fluid pipe 1 can be maintained at a specific frequency of higher order selectively from a plurality of the natural frequencies of the fluid pipe 1.
When a setting is made to oscillate at the basic natural frequency, the fluid pipe 1 vibrates while warping up and down in the order of the one-dot chain line L1, the solid line L0, and the two-dot chain line L2 of FIG. 6A by the oscillation.
On the other hand, by the oscillation and the flow of the measurement fluid 100 (FIG. 4), a force known as Coriolis force acts on the fluid pipe 1, whereby the fluid pipe 1 warps and vibrates up and down while being twisted as shown in FIG. 6B.
The magnitude of the Coriolis force is proportional to the mass of the fluid flowing within the fluid pipe 1, the velocity thereof, and the angular velocity of oscillation, and the direction of the force coincides with the direction of the vector product of the movement direction (velocity vector) of the fluid and the angular velocity at which the fluid pipe 1 is oscillated. Since the flow directions of the fluid will be opposite to each other between the inlet side and the outlet side of the fluid in the fluid pipe 1, the forces acting on the two straight pipe portions will be opposite to each other in the up-and-down direction. For this reason, a torque of twist is generated in the fluid pipe 1 by the Coriolis force. This torque changes with the same frequency as the oscillation frequency, and the amplitude value thereof will have a predetermined relationship with the mass flow rate of the fluid.
The warpage of the fluid pipe 1 of FIG. 6A by the oscillation and the twist of the fluid pipe 1 of FIG. 6B by the Coriolis force are superposed on each other. The calculator 32 of the microcomputer 3 of FIG. 5 calculates the mass of the measurement fluid 100 that passes through the flow path of the fluid pipe 1 based on the phase of the amplitude of the twist, that is, the phase difference of the velocity signals of vibration at respective positions constituting information of each vibration detected by the detectors 2A, 2B.
Here, there is an electric potential difference between the fluid pipe 1 or the permanent magnet 23 and the coil 22 of FIG. 4, and also the fluid pipe 1 and the coil 22 are close to each other. Therefore, when the fluid pipe 1 is made of a material having electric conductivity such as a metal, a noise may enter the detection signal by the capacitance coupling in some cases. In a general electronic appliance, the noise of the detection signal can be removed by a low-pass filter.
However, in a mass flow meter of Coriolis type, the flow rate is calculated by the phase difference between the two detection signals as described above, and this phase difference is a minute value. For this reason, even if the cut-off frequency of each low-pass filter is larger by several digits than the frequency of the vibration of the fluid pipe, the variation in the resistors and the capacitors constituting the two low-pass filters as well as the temperature characteristics have a great influence on the measurement precision. Therefore, the measurement precision decreases by the noise removal using the low-pass filter. Hereafter, this will be more specifically described.
For example, a study will be made in a case where the noise is removed by the low-pass filters in the two detection circuits 33, 33 of FIG. 5. Now, assuming that the cut-off frequency of the low-pass filters is 100 kHz, the damping at 1 MHz will be 20 dB.
Assuming that the fluid pipe vibration is 500 Hz and that the resistor or the capacitor of one low-pass filter is shifted by 5% from the other low-pass filter, the shift will be 0.25 mrad. Assuming that the full scale of the measurement range is 10 mrad, a shift of 2.5% will be generated relative to the full scale. When the cut-off frequency of the filters is lowered, this shift will further increase.
Due to these reasons, the decrease in the measurement precision cannot be prevented by the noise removal using the low-pass filter, so that the noise generation itself needs to be prevented. Thus, it can be considered that by short-circuiting between the ground in the inside of the detection circuit 33 and the fluid pipe 1 as depicted by the broken line shown in FIG. 7A to allow the two to have the same electric potential, the decrease in the measurement precision caused by the noise is prevented.
However, though this solution method can be adopted when the minus side of an external power source 50 of a user is grounded as shown in FIG. 7A, the method cannot be adopted in the case of the positive grounding in which the plus side Vcc of the external power source 50 is grounded as shown in FIG. 7B.
That is, in FIG. 7B, the minus side of the external power source 50 is short-circuited to the fluid pipe 1 via a power source line to the detection circuit 33, and is further grounded via a main pipe 51 of the fluid pipe 1. Therefore, when the plus side Vcc of the external power source 50 is grounded as shown by the two-dot chain line, the plus side Vcc and the minus side of the external power source 50 will be short-circuited.
On the other hand, since both of the minus grounding and the plus grounding are present for the user side, it is difficult for a manufacturer to adopt this solution method.
Therefore, in the conventional cases, in order to meet the plus grounding as well, a method is adopted in which an insulated power source is incorporated or an insulated power source is used as an external power source. For example, as shown in FIG. 8, when an electric power is supplied to the detection circuit 33 in a state of being insulated from the external power source 50, the situation as described above where the plus side Vcc and the minus side of the external power source 50 are short-circuited will not occur even if the detection circuit 33 is grounded and the plus side Vcc of the external power source 50 is grounded.
However, in the case of supplying an electric power to the detection circuit 33 in an insulated state, the whole circuit will be complex, and the costs will rise in accordance therewith. The output signals and the like need to be output in an insulated state, and it causes an obstacle against the cost reduction of the flow meter.
Such a problem occurs not only in the detectors but also in the oscillators as well. That is, the vibration state of the oscillation will not be in an expected state due to the noise that has penetrated into the coil of the oscillator, thereby inviting decrease in the measurement precision.
Here, though WO91/08448 (abstract) discloses a magnetic shield to enhance the efficiency of the magnetism of a coil, no disclosure is given as to the shielding so as not to generate a noise in the coil.
Therefore, an object of the present invention is to enable electric power supply in a non-insulated state from an external power source in a Coriolis type mass flowmeter irrespective of whether the external power source is positively grounded or negatively grounded, and to prevent decrease in the measurement precision by preventing the generation of noise in the coil.